When EhloMail Collaboration Server is installed, the time zone is configured and set in the default COS. This time zone should be the time zone that the majority of users in the COS are located. If users are distributed such that there is no common time zone then set or have the users set, the time zone for each account.
To change the time zone either in the Configure > COS or Manage > Accounts go the Preferences page and in the Calendar Options section, select the appropriate time zone.